Defective nozzle compensation

ABSTRACT

An image recording apparatus ( 2000 ) is disclosed, which comprises forming elements ( 2002 ) for forming an image, a memory ( 2006 ) indicating relative desirability of utilising the forming elements ( 2002 ) for forming the image, and processing means ( 2008 ) for computing image recording signals using input image signals ( 2012 ) and the stored data. The use of a particular forming element is thereby biased dependent upon the relative desirability data for the other forming elements and the corresponding input image signals for the other forming elements.

This application is a division of application Ser. No.09/859,437, filedMay 18, 2001, the entire disclosure of which is incorporated herein byreference.

COPYRIGHT NOTICE

This patent specification contains material that is subject to copyrightprotection. The copyright owner has no objection to the reproduction ofthis patent specification or related materials from associated patentoffice files for the purposes of review, but otherwise reserves allcopyright whatsoever.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to printers, and in particularto ink jet printers.

BACKGROUND ART

Digital image printing systems which use multiple ink nozzles integratedwithin a print head have increased rapidly in popularity in recentyears. Either at manufacture, or during operation, the particularprinting nozzles within a print head can fail to deliver ink droplets ofthe required size, or alternatively, with the required positionalaccuracy. These problems can arise from a combination of manufacturingflaws, and wear and tear. In extreme cases, individual nozzles can failto deliver any ink whatsoever, due to being blocked or damaged.

Defective nozzles, be they blocked or merely defective in terms of theirperformance, can be identified manually or automatically by examiningtest print output.

Even where nozzles delivery ink droplets satisfactorily, the nozzles canvary in their characteristics, and produce uneven print densities. U.S.Pat. No. 5,038,208 entitled “Image Forming Apparatus with a Function forCorrecting Recording Density Unevenness” describes a method andapparatus related to improving the evenness of print density produced bynozzles of varying characteristics. The patent discloses a printerhaving a multi-nozzle printing head, this printer also storing dataassociated with the image-forming characteristics of each of themulti-nozzle heads. Furthermore, a corrector means for correcting theimage forming signals based upon the data stored in memory correspondingto the multi-nozzle head characteristics is disclosed. Theaforementioned correction is performed either by retrieving a correctedvalue from a memory, or by retrieving a correction value from a memory,and adding the correction value to the uncorrected image value. In bothcases, the image correction data is accessed using both the uncorrectedimage value, and a nozzle counter value. This type of correction isknown as “head shading”. Head shading methods do not, however,satisfactorily eliminate print artefacts where a blocked nozzle ispresent, or where the ink ejection performance of a nozzle fails to meetminimum requirements.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to substantially overcome, orat least ameliorate, one or more disadvantages of existing arrangements.

According to a first aspect of the invention there is disclosed amethod, in printing an image comprising a plurality of image values, ofcompensating for one or more defective printer nozzles in a plurality ofprinter nozzles, said method comprising the steps of:

-   -   biasing, for each first image value associated with a first        nozzle, at least one second image value associated with another        nozzle, said biasing being dependent upon said first image value        and a printing desirability factor for said first nozzle;    -   halftoning said at least one biased second image value to form        at least one corresponding nozzle firing value; and    -   printing the image using said at least one nozzle firing value,        thereby reducing print artefacts otherwise caused by the one or        more defective nozzles.

According to another aspect of the invention there is disclosed an imagerecording apparatus for recording an image comprising a plurality ofimage values, the apparatus comprising:

-   -   a plurality of forming elements for forming the image using        image recording signals;    -   memory means for storing data for said forming elements        indicating the relative desirability of utilising said forming        elements for forming the image;    -   image processing means for computing biased image values using        said image values and said data stored in said memory means,        wherein use of a particular forming element is biased dependent        upon the relative desirability data of other forming elements;        and    -   halftoning means for halftoning the biased image values to form        corresponding said image recording signals.

According to another aspect of the invention there is disclosed an imagerecording apparatus for recording an image comprising a plurality ofimage values, the apparatus comprising:

-   -   a plurality of forming elements for forming the image using        image recording signals;    -   memory means for storing data for said forming elements        indicating the relative desirability of utilising said forming        elements for forming the image;    -   image processing means for computing biased image values using        said image values and said data stored in said memory means,        wherein use of a particular forming element is biased dependent        upon the relative desirability data of other forming elements        and the image values for the other forming elements; and    -   halftoning means for halftoning the biased image values to form        corresponding said image recording signals.

According to another aspect of the invention there is disclosed an imagerecording apparatus for recording an image comprising a plurality ofimage values, the apparatus comprising:

-   -   a plurality of forming elements for forming the image using        image recording signals;    -   memory means for storing data for said forming elements        indicating the relative desirability of utilising said forming        elements for forming an image;    -   image processing means for redistributing said image values        based on said data stored in said memory means so as to form        redistributed image values the use of which biases use of said        forming elements; and    -   halftoning means for halftoning the redistributed image values        to form corresponding said image recording signals.

According to another aspect of the invention there is disclosed an imagerecording apparatus for recording an image comprising a plurality ofimage values, the apparatus comprising:

-   -   a plurality of forming elements for forming the image using        image recording signals;    -   memory means for storing data for said forming elements        indicating the relative desirability of utilising said forming        elements for forming the image;    -   image processing means for redistributing said image values        based on said data stored in said memory means to form        redistributed image values the use of which biases the use of        said forming elements, wherein the use of a particular one of        said forming elements is thereby biased dependent upon the        relative desirability data of other forming elements and the        image values for the other forming elements; and    -   halftoning means for halftoning the redistributed image values        to form corresponding said image recording signals.

Other aspects of the invention are also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

A number of preferred embodiments of the present invention will now bedescribed with reference to the drawings, in which:

FIG. 1 depicts a prior art halftoning arrangement;

FIG. 2 illustrates a prior art unevenness correction, and halftoningarrangement;

FIG. 3 shows the unevenness correction process in FIG. 2 in more detail;

FIG. 4 depicts an arrangement for defective nozzle compensation inaccordance with a preferred embodiment of the present invention;

FIG. 5 shows a full-width fixed head printer whereupon the arrangementin FIG. 4 can be applied;

FIG. 6 shows a shuttle-head printer, to which the arrangement in FIG. 4can be applied;

FIG. 7 illustrates redistribution of image values from a defectivenozzle to neighbouring non-defective nozzles in a first embodiment ofthe invention;

FIG. 8 shows a process for redistribution of image values in accordancewith the arrangement illustrated in FIG. 7;

FIG. 9 illustrates a pattern of printed dots in a single colour which isoutput by the arrangement in accordance with FIG. 7;

FIG. 10 shows defective nozzle compensation using cross-colourcompensation according to a first embodiment of the present invention;

FIG. 11 provides an example of printable dots-per-nozzle for a pixel inaccordance with a second embodiment of the present invention;

FIG. 12 illustrates “full” paper coverage by dots of a single colourboth with and without defective nozzles being present;

FIG. 13 shows the application of quantisation correction in a preferredembodiment of the present invention;

FIG. 14 illustrates tuning a three level halftone error diffusiontransfer function;

FIG. 15 depicts a process for printing a halftoned image using a tunederror diffusion table;

FIG. 16 depicts a prior art error diffusion arrangement;

FIG. 17 shows an untuned 3 level halftone error diffusion table:

FIG. 18 shows a tuned 3 level halftone error diffusion table;

FIG. 19 depicts a general purpose computer upon which preferredembodiments of the invention can be practiced; and

FIG. 20 depicts a block diagram representation of an apparatus forredistribution of image values from a defective nozzle to neighbouringnon-defective nozzles

DETAILED DESCRIPTION INCLUDING BEST MODE

Where reference is made in any one or more of the accompanying drawingsto steps and/or features, which have the same reference numerals, thosesteps and/or features have for the purposes of this description the samefunction(s) or operation(s), unless the contrary intention appears.

The principles of the preferred methods described herein have generalapplicability to printing devices which have a multiplicity ofprint-forming means integrated into a single printing head. However, forease of explanation, the steps of the preferred methods are describedwith reference to ink jet printers. It is, however, not intended thatthe present invention be limited thereto.

Arrangements of the present invention are applied to image printingsystems, having recording heads each of which has an array of recordingelements, and which uses data describing defective recording elements inorder to provide an improved printer output. For ease of illustration,the description is directed to multiple print heads each having a lineararray of ink jet nozzles, however the embodiments can be extended toother types of printing systems.

A common aspect of the apparatus and methods encapsulated in thedescribed embodiments is the use of defective nozzle data. This is usedfor reducing the use of defective nozzles, and for correspondinglyincreasing use of non-defective nozzles which print in the vicinity ofthe positions at which the defective nozzles print. This has the effectof reducing print artefacts caused by the defective nozzles. Whereinter-nozzle spacing is sufficiently small, the aforementionedredistribution of nozzle use, or “cross-nozzle” compensation, results insignificant reduction in artefacts. Accordingly, print heads which areeither manufactured with defective nozzles, or which develop defectivenozzles over time, can continue to be used while producing good qualityoutput.

One approach for providing cross-nozzle compensation is to reduce imagevalues associated with defective nozzles, correspondingly increasing theimage values associated with neighbouring, non-defective nozzles. Thiscan be achieved by a image redistribution process which is furtherdescribed in relation to the first and second embodiments (eg see FIGS.4 and 8). In colour printing, an additional approach for providingcross-nozzle compensation is available. This is to compensate for adefective nozzle of a first colour component by increasing an imagevalue associated with a corresponding nozzle of a second colour, ie onewhich prints at the same position as the defective nozzle. This isdescribed further with respect to the first embodiment (eg see FIG. 10).

In a first arrangement, image value redistribution is performed in amanner which restricts the redistribution so that resultant image valuesof the receiving nozzles (ie., the neighbouring nozzles), remain withintheir “normal” range. The “normal” range of a printer nozzle isconsidered to be that range which, during normal operation, supports theprinting of intensity values between “0” and “maximum”, or in 8-bitnumeral terms, between 0 and 255. In the first embodiment, since thereis a restriction on the amount of error which can be redistributed toneighbouring nozzles, in general the complete image value assigned to adefective nozzle will not always be able to be completely redistributed.This can be overcome, by using the residual, ie undistributed, imagevalue to perform compensation by manipulating the image values of othercolour components.

In a second arrangement, image value redistribution is performed in amanner which allows the receiving nozzles to exceed their normal range,and an increased, (“superintensity”) ink ejection per colour componentper pixel is permitted, beyond that which is required to provide normalfull ink coverage of the paper. The second embodiment also discloses amethod of tuning how often the superintensity ink ejection is provided.

In an ink-jet printer, image data is typically converted to nozzlefiring control data. The image data is typically multi-tone colour datawhich must be converted into nozzle firing data in order to control theink ejection of the nozzles of the print heads.

The nozzle firing data for a multi-tone colour image can be consideredas a colour image with a reduced number of colour intensity values perpixel. The process of determining image data with a reduced number ofintensity values per pixel is referred to as “halftoning”.

For ease of explanation, a rectangular array of pixels is assumed, witheach pixel having an intensity value for each one of the colourcomponents Cyan, Magenta, Yellow and Black. Equivalently, the multi-tonecolour image can be considered as a set of four rectangular arrays ofintensity values, one array per colour, each array having the samephysical dimensions. Intensity values are represented as 8-bit numbers,having a value in the range of 0 to 255. Value 0 corresponds to aminimum colour density (i.e., no deposition), and the value 255corresponds to a maximum colour density (i.e., full ink deposition).

Nozzle firing data is generated from the aforementioned image data, andcomprises an array of nozzle firing values, one array for each colourcomponent.

Again for ease of explanation, for each colour component, the dimensionsof the nozzle firing array are assumed to be the same as the dimensionsof the corresponding array of multi-tone image intensity values.Accordingly, it is assumed that there is a one-to-one correspondencebetween multi-tone image pixels, and positions in a nozzle firing array,so that the description is not burdened by spatial resolution conversionissues. In the event that nozzle separation in a print head is differentto the input image pixel spacing, image values for nozzles can typicallybe derived from pixel image values by interpolation, or by resolutionconversion algorithms. After such conversion, defective nozzlecompensation can then be applied to the nozzle image values.

Typically, for each colour component, there is one nozzle firingposition for each position in a nozzle firing array. This is not alwaysthe case however. For example, in the second embodiment, there are twonozzle firing positions corresponding to each position in a nozzlefiring array.

In the first arrangement, each nozzle firing value is a 1-bit number,where the value “0” indicates that ink should not be ejected for thatposition in the nozzle firing array, and a value “1” indicates that inkshould be ejected. In the second embodiment, each nozzle firing valuemay be 0, 1 or 2, this value indicating the number of ink droplets to beejected for that position in the nozzle firing array.

FIG. 1 depicts a prior art halftoning arrangement. A multi-tone 8 bitimage value 100 is input into a halftoning process 102, which produces anozzle firing value 104. The halftoning process is often described as a“binarisation” process, since in the present case it generates datadescribing whether a nozzle should fire or not at each nozzle firingposition. Halftoning may be performed by error diffusion or dithering,and typically, each colour component is halftoned independently.

In FIG. 2 the 8-bit multi-tone image array value 100 is processed by anunevenness correction process 200 prior to the halftoning process 102.This optional prior art arrangement is described in the cited U.S. Pat.No. 5,038,208.

FIG. 3 provides more detail about the aforementioned unevennesscorrection process. Each nozzle is associated with a curve whichcharacterises its performance. This curve is a mapping, for each nozzle,from an uncorrected 8-bit image value, to an unevenness-corrected 8-bitimage value. A nozzle counter value 300 is used to retrieve a curvevalue from a nozzle curve table 302. The curve value 304 which isselected and output is processed in accordance with the 8 bit multi-toneimage array value 100 in a curve table process 306. This process 306produces an unevenness corrected image value 202.

FIG. 4 illustrates a defective nozzle compensation arrangement. The 8bit multi-tone image array value 100 is input into a defective nozzlecompensation process 400. This process 400 outputs a redistributed imagevalue 402. This redistributed value 402 has the same precision, i.e.,8-bits, as the multi-tone image array value 100. Thereafter, theredistributed image value 402 is input into an unevenness correctionprocess 200, and subsequently, into the halftoning process 102, in orderto produce the final processed nozzle firing array value 104. It isnoted that the defective nozzle compensation process 400 is performedprior to the unevenness correction process 200, and prior to thehalftoning process 102. Alternatively, defective nozzle compensation 400can be performed prior to the halftoning process 102, and the unevennesscorrection process 200 can be omitted.

FIG. 5 illustrates a full-width fixed head printer, in which a sheet ofpaper 500 having a width 502 and being fed in a direction depicted by anarrow 504 passes beneath a full width print head 506. A segment 514 ofthe stationary print head 506 is shown in more detail in an inset 508,where it is seen to comprise a plurality of individual print nozzles510. A print head for each colour component is provided, each of theseheads being oriented at right angles to the paper feed directiondepicted by the arrow 504. The print head 506 is fixed, and ink isejected from each of the print head nozzles, e.g., 510, while the paperis advanced underneath the print head. An entire row (i.e., scanline) ofthe image is printed at a time. An individual printer nozzle, e.g., 510,is restricted to printing dots of a column of pixels, since the printhead 506 is fixed in position.

FIG. 6 illustrates a shuttle-head ink jet print system, in which a sheetof paper 600, having a width 602, is fed in a direction depicted by anarrow 604. A shuttle print head 612 moves at right angles across thepaper 600, as depicted by a bi-directional arrow 610. A segment 620 ofthe print head 612 is shown in more detail 606 in an inset 618. Thisshows that the print head 612 comprises a plurality of individualprinter nozzles 608. A print head 612 is provided for each colourcomponent, and each print head 612 is oriented parallel to the paperfeed direction as depicted by the arrow 604. The print heads 612 moveover the paper at right angles 612 to the paper feed direction 604, withink being ejected from the print head nozzles 608 while the paper 600 isstationary, and the print heads 612 are performing a shuttle scan acrossthe page 600. A band of scanlines is printed in one pass of the shuttleprint heads. The width of the band of scanlines printed in a single passof the shuttle print head 612 is restricted by the length of the printhead 614. In any given pass of the shuttle print heads, an individualnozzle, 608 is restricted to printing dots of a particular scanline(i.e., a row) of pixels. Between passes of the shuttle print head 612,the paper 600 is advanced in the paper feed direction 604, so thatsuccessive bands of scanlines can be printed. Typically, to reduceprinter artefacts due to nozzle inconsistencies, the paper 600 isadvanced in a direction 604 by only a fraction of the length 614 of theband of scanlines printed by one pass of the shuttle print head. In thismanner, each scanline is printed using multiple passes of the shuttleprint head 606. Dots of a colour component of a scanline are accordinglyprinted by a different nozzle of the same print head for each pass.

In both styles of printer, i.e., the full-width fixed head printer (seeFIG. 5), and the shuttle-head printer (see FIG. 6), the relativemovement between the print heads (506, 612) and the paper (500, 600) isat right angles to the respective print head. Each nozzle (510, 608)prints a line of dots. Neighbouring nozzles print neighbouring lines ofdots.

FIG. 7 illustrates image value distribution from a defective nozzle toneighbouring non-defective nozzles. A line of nozzles 700 to 702 withina print head (not shown) is depicted. A defective nozzle 704 isindicated by an “X” i.e., 706. A line of rectangular pixels 708 to 710is shown adjacent to the line of nozzles 700 to 702. The aforementionedrepresentation is equally applicable to a full-width fixed head printer,and to a shuttle head printer arrangement. A graph with axes of imagevalue 712 against pixel number 714 illustrates a desired sequence ofdescending image values 716 to 724. These are the assigned image valueswhich are desired to be printed by the print head. Since the nozzle 704is defective, the desired image value 720 cannot be printed. The graphof actual image value 726 against pixel number 728 illustrates theactual printed image values after image value redistribution. It isnoted that the image value 730 which corresponds to the defective nozzle704 has a 0 image value, while the immediately neighbouring pixels haveimage values 732 and 734, these having being increased in order tocompensate for the 0 image value 730. The result of redistributing theimage value from the defective nozzle 704 to the neighbouring nozzles736 and 738 is that dots which would otherwise have been allocated tothe defective nozzle, were it not defective, are instead printed by thenozzles 736, 738 which are situated on either side of the defectivenozzle 704.

FIG. 8 shows a process for redistribution of image values in accordancewith the arrangement described in regard to FIG. 7. In a first step 1802of the process 1800, data corresponding to relative desirability ofusing various forming elements are determined and stored. Thereafter, ina step 1804, an input image signal for a current nozzle is input. In afollowing decision step 1806, a determination in made, depending forexample, upon a measure of desirability for the current nozzle, whetherbias is required. If bias is indeed needed, then the process 1800 isdirected in accordance with a “yes” arrow to a step 1808 in which aninput image signal for another nozzle is input. In a following step1810, some or all of the input image signal for current nozzle isdistributed to the other nozzle, ie. the input image signal for thecurrent nozzle is added to the input image signal for the other nozzle.Thereafter, in a step 1812, the current nozzle firing data is generated.In a following step 1814, an index for the current nozzle isincremented, after which the process 1800 is returned to the step 1804.Returning to the decision step 1806, if the decision step determinesthat bias is not after all required, then the process 1800 is directedin accordance with a “no” arrow to the step 1812, which generates thenozzle firing data. It is noted that the initial step 1802, in whichrelative desirability data for the various forming elements are stored,is performed only at the outset of the process 1800.

FIG. 9 depicts a patterns of dots of a single colour component which areto be printed according to values in a nozzle firing array. Dots can beprinted at positions on a rectangular grid 800, which consists ofvertical grid lines 804 and horizontal grid lines 802. In the figure,dots have been shown in outline only, eg. 806, so that the pixel grid800, remains visible. The rectangular grid 800 allows for one dot pergrid position. All dots of a particular grid column are printed by thesame nozzle. In the figure, a nozzle corresponding to the centre columnof pixels 812 is defective. The illustrated dot outlines correspond to anozzle firing value array generated using defective nozzle compensationby image value distribution, followed by halftoning, for an image regionof near 50% intensity. As a result of the defective nozzle compensationby image value distribution, no dots are printed in grid column 812, andaccordingly, additional dots 808, 810 and 811 are printed in theneighbouring grid columns. Due to the additional dots 808, 810 and 811,average ink deposition near the blocked nozzle is not reduced, andaccordingly, the 50% intensity of the image values is reproduced in theaverage ink deposition. Advantageously, this desired average inkdeposition is maintained with reduced print artefacts due to thedefective nozzle. This advantageous performance is maintained both wherethe defective nozzle is blocked, or alternatively, where it is defectiveand, for example, ejects ink unreliably, too far to one side or thelike.

A C programming language code fragment is now presented in relation to afirst embodiment of the present invention, i.e., defective nozzlecompensation by restricted image value redistribution for a full-widthfixed head printer. For ease of explanation, it is assumed that (i)there is one nozzle per pixel of a scanline, and (ii) the defectivenozzle data consists of a one-bit value for each nozzle, indicatingwhether or not the associated nozzle is defective, and (iii) imageintensity is redistributed using only nearest neighbour nozzles, i.e.,pixels. clamp_val = 255; .. /*  The following processing is performedone pixel at a time  (ie one nozzle at a time)  from the first pixel ofa scanline to the last. */ /*  if the current pixel's nozzle isdefective  and the next pixel's nozzle is not defective  then distributeas much as possible of remaining intensity  of current pixel to nextpixel */ if (defective[n] && !defective[n + 1]) {  old_val = image[n +1];  new_val = image[n + 1] + image[n];  if (new_val > clamp_val)  {  new_val = clamp_val;  }  image[n + 1] = new_val;  image[n] −= new_val− old_val; } /*  if the current pixel's nozzle is not defective  and thenext pixel's nozzle is defective  then distribute as much as possible ofhalf intensity  of next pixel to current pixel */ if (!defective[n] &&defective[n + 1]) {  old_val = image[n];  new_val = image[n] + image[n +1] / 2;  if (new_val > clamp_val)  {   new_val = clamp_val;  }  image[n]= new_val;  image[n + 1] −= new_val − old_val; }

Prior to image value re-distribution, the 8 bit image values fall intothe range 0 to 255, and accordingly, image values of 0 and 255 representan unbiased operating range of 0% and 100% respectively. The above codeassociated with the first embodiment restricts the dynamic range afterre-distribution to the same range ie 0 to 255, and accordingly, imagevalues of 0 and 255 represent a biased operating range of 0% and 100%respectively.

It is found, for the case where nozzle separation is equal to 1/600 ofan inch, that restricted image value redistribution as described abovesignificantly reduces streaking artefacts which typically occur in thepresence of a blocked or non-firing nozzle.

When the image region to be printed consists of a constant image valuewhich is less than or equal to (⅔) *255, all of the image value of thenon-firing nozzle is, by virtue of the aforementioned image value, ableto be distributed to immediately neighbouring nozzles. For such imageregions, defective nozzle compensation using restricted image valueredistribution as described previously, provides a clear reduction inthe streaking artefact which typically occurs as a consequence of anon-firing nozzle.

In contrast however, where the image region to be printed consists of aconstant image value greater than (⅔) *255, then some of the image valueof the non-firing nozzle is unable to be distributed to its immediatelyneighbouring nozzles. In such cases, a streak due to the non-firingnozzle remains visible even after defective nozzle compensation usingthe restricted image value redistribution as described.

In summary, defective nozzle compensation using restricted image valueredistribution is limited by the range restriction of the immediatelyneighbouring print nozzles. This means that a residual image value,equal to the amount which could not be redistributed, is “retained” bythe blocked nozzle.

The concept of image redistribution can, however, be extended as willnow be described. In a multi-colour printing arrangement, theaforementioned residual image value, ie. the value which cannot beredistributed to immediately neighbouring nozzles, can nonetheless beused for “cross-colour” compensation. Cross-colour compensation relatesto the fact that print artefacts which may normally be produced as aresult of a blocked nozzle of a first colour component, can often bereduced by printing dots of another colour component, in the area wheredots of the first colour component are missing. In other words,cross-colour compensation is the use of other colour components toreduce artefacts due to defective nozzles in a first colour.

FIG. 10 shows how defective nozzle compensation using restricted imagevalue redistribution as previously described can be extended toincorporate cross-colour compensation for the Cyan and Magenta colourcomponents where either, or both the print heads associated with Cyanand Magenta may have blocked nozzles. FIG. 10 shows an 8-bit Cyan imagearray value 900 being input into a restricted image value redistributionprocess 902. Similarly, an 8-bit Magenta image array value 910 is inputinto a restricted image value redistribution process 912. Thereafter,the 8-bit redistributed image values 904 and 914 respectively are fedinto a cross-colour compensation process 906. The cross-colourcompensation process 906 makes use of cross-colour compensation tocorrect for artefacts caused within each of the Cyan and Magenta images,resulting from respective blocked nozzles in each separate print head.The cross-colour compensation process 906 outputs an 8-bit Cyan nozzlecompensated image value 908, and an 8-bit Magenta nozzle compensatedimage value 916.

The processing performed in the cross-colour compensation process 906,this being performed on a per pixel basis for both Cyan and Magenta, isdescribed in the following C language code fragment: /*  if Cyan nozzleis defective and Magenta nozzle is not defective  then augment theMagenta image value */ if (cyan_defective[n] && !magenta_defective[n]) { new_val = magenta[n] + f1 * cyan[n];  if (new_val > 255)  {   new_val =255;  }  magenta[n] = new_val; } /*  if Cyan nozzle is not defective andMagenta nozzle is defective  then augment the Cyan image value */ if(!cyan_defective[n] && magenta_defective[n]) {  new_val = cyan[n] + f2 *magenta[n];  if (new_val > 255)  {   new_val = 255;  }  cyan[n] =new_val; }

The aforementioned C code provides Magenta colour compensation for adefective Cyan nozzle, or vice versa, on a per pixel basis. The valuesof the parameters f1 and f2 are determined by experiment. For 600 dotsper inch (dpi) printing, values f1 and f2 equal to 0.2 provide goodreduction of streaking artefacts due both to (i) a non-firing Cyannozzle in an image region of high Cyan and low Magenta values, and (ii)a non-firing Magenta nozzle in an image region of low Cyan and highMagenta values.

Since the Cyan/Magenta cross-colour compensation for a non-firing Cyannozzle is effective in high Cyan image regions, and since restrictedimage value distribution is effective for image regions up to ⅔ maximumimage value, it is seen that these two methods compliment each other.Accordingly, for 600 dpi printing, the combination of restricted imagevalue redistribution, followed by Cyan/Magenta cross-colourcompensation, provides a good reduction in streaking artefacts due toeither a blocked Cyan nozzle or a blocked Magenta nozzle, over most ofthe printer gamut.

A second arrangement is now proposed to improve the effectiveness ofdefective nozzle compensation by image value redistribution, this timethrough the use of print systems which can eject more ink per nozzlethan is typically required to achieve a full ink coverage of the paper.In other words, this embodiment relates to extending the range of aprint nozzle beyond the 0-255 normal boundaries. This type of printingwill hereafter be referred to as super output intensity printing.

FIG. 11 shows an example of dots printable per nozzle per pixel for asingle colour component for super output intensity printing. A pixelgrid 944 comprising adjacent rectangular pixel positions 946 typicallyprovides for pixel separation of 1/600 inch in both directions 948.Super output intensity printing, however, enables dots to be printedwith half the aforementioned separation, ie., 1/1200 inch, as shown bydots 950, 952, which are separated by 1/1200 inch as indicated by arrows940, 942. In the present example, print nozzles are separated by 1/600inch along the print head, however each nozzle can fire twice duringrelative movement of the head and the paper, through 1/600 inch.

Nozzle firing data associated with pixels to be printed can berepresented as an array for each colour component, the array comprisingvalues in the range 0 to 2. Each value thus represents nozzle firingdata for a particular colour component, and for a particular pixel. Thevalue (ie. halftone output level) “0” corresponds to no dots printed forthe pixel; value “1” corresponds to 1 dot printed for the pixel, and thevalue “2” corresponds to 2 dots printed for the pixel. Since the printsystem is designed to achieve full ink coverage of the paper bydepositing 1 dot per pixel position 946, and the print system is able toeject two dots per pixel position, it is possible to ensure that theaverage volume of ink deposited in the vicinity of a nozzle is notreduced when a single nozzle is not firing.

FIG. 12 shows a pixel grid 1104 fully covered, in the ideal situation,by individual dots 1102. In the event that a print nozzle is blocked,and thus unable to fire in a column depicted by an arrow 1108, superintensity printing is used to produce additional printed pixel dots1110, thus maintaining the required average ink deposition in the regionof the unprintable column 1108. The actual printed dot pattern 1106 isan approximation to the desired printed dot pattern 1100. Superintensity output printing also takes advantage of the fact that printeddots typically swell when they overlap, and consequently, in superintensity output printing 1106, increased dot swelling can be expectedin the region of the super intensity output dots 1110, thereby reducingthe area of paper uncovered by ink 1108.

FIG. 12 describes a particular method of performing super outputintensity printing, this being by ejecting an extra ink droplet perpixel per colour component. This is depicted by overlapping dots 950,952 having a spacing half the normal spacing 940, 942 (see FIG. 11).

Alternatively, more than one extra droplet per pixel position may beejected, larger ink droplets may be ejected, or a combination of theaforementioned methods may be used.

The use of super intensity output printing results in full, or completeimage value redistribution. This differs from restricted image valueredistribution, in that the redistribution is not limited by the normalrange of the pixels receiving the redistributed image values. The imagevalues of the receiving pixels can accordingly be increased beyond theirnormal maximum boundary values. Assuming that (i) there is one nozzleper pixel of a scanline, (ii) defective nozzle firing data consists of a1 bit value for each nozzle, indicating whether or not the associatednozzle is defective, and (iii) image intensity is redistributed usingonly nearest neighbour pixels, then the C code relating to restrictedimage value redistribution which has been provided is applicable, exceptthat the value used to clamp resulting image values is changed to 510(ie., twice the normal level of 255, and accordingly, image values of 0and 510 represent a biased operating range of 0% and 200% respectively).Given this new clamping value, and where there is a single defectivenozzle, the total image value associated with the defective nozzle canbe redistributed to pixels corresponding to neighbouring nozzles. Theimage values of such neighbouring pixels will now fall in a range of 0to 510, which requires 9 bits for representation. Accordingly, theoriginal 8 bit input image value, eg. 100 in FIG. 1, has now beenincreased to a 9 bit image value after full image value redistribution.It is desirable to map the resultant 9 bit image values back to theoriginal 8 bit dynamic range, in order to avoid modifying one or more ofthe unevenness correction process 200, and the halftoning process 102(see FIG. 4). In particular, if the number of bits required to representimage values input to the unevenness correction process 200 is increasedfrom 8 to 9 bits, then that process 200 requires either a two-foldincrease in the size of the curve tables, or alternatively, more complexprocessing.

FIG. 13 shows an 8 bit multi-tone image array value 1200 being inputinto a full image value redistribution process 1202, which outputs afully redistributed image value array 1204 having a 9 bit range. Thisoutput 1204 is input to a checkerboard quantisation process 1206, whichreduces the 9 bit range of image values 1204 back to an 8 bit fullyredistributed and quantised image value 1208 having a range of 8 bits.The checkerboard quantisation process 1206 effectively divides the imagevalue 1204 by 2, alternately rounding fractional image values up, ordown. The rounding is performed alternately according to a checker boardpattern. Accordingly, if the sum of (i) the pixel scanline number, and(ii) the pixel position within the scanline, is even, then the roundingis opposite to the rounding performed when the sum is odd. Thecheckerboard quantisation process also includes at least one instance ofspecial case processing, this occurring if the input image value is 255,in which case the output image value is always rounded in the samedirection. This special case processing is provided because imageregions having an input value of 255, ie. the maximum input image value,should typically result in a halftone output corresponding to full inkcoverage of paper. If this special case processing is not included, thenthe resultant halftone output for such image regions is a mix ofhalftone output levels, this non-uniform pattern being undesirable forimage regions having a constant maximum input image value.

The checkerboard quantisation process 1206, which rescales the range ofthe image value 1204 from 9 bits back to 8 bits (1208), provides anumber of advantages over simple truncation of image values to the mostsignificant 8 bits. Although both checkerboard quantisation and simpletruncation result in a range of 0 to 255 for a pixel, checkerboardquantisation produces a greater number of different resulting localaverage image values for pixels of a region, given an image region ofconstant input image value. For checkerboard quantisation therefore, aregion of an input image which is a gradual blend from one image valueto another, is represented by a smoother transition in local averageimage values. This reduces colour step artefacts which typically occurfor simple truncation.

Super intensity output printing, which as described requires depositionof additional ink, can cause problems including unwanted ink swelling,increased paper wetting, and increased ink drying time. In order tooptimise, or maximise the beneficial effect of super intensity inkdeposition in the vicinity of a blocked nozzle, it is sometimesdesirable that the frequency of super intensity ink deposition per pixelbe reduced by tuning it down.

In contrast, another problem which can be associated with the use ofsuper intensity output printing, is that if sufficient super intensityink is not deposited on the paper often enough, then the unprinted lineresulting from a defective printing nozzle will be insufficientlycompensated for. It is thus advantageous, in this case, to tune thefrequency of super intensity ink deposition per pixel, typicallyincreasing the frequency in this instance.

FIG. 14 illustrates the aspect of tuning in the context of a three levelhalftone error diffusion process. The halftoning process is representedby a graph having an ordinate 1400 representing the average number ofoutput dots printed (ie. 0, 1 or 2 dots per particular pixel), and anabscissa 1402 representing the input image value, ie. from 0 to 255 forsuper intensity output printing after checkerboard quantisation. Therelationship, or transfer function, between the input image value andthe average three level halftone output is typically depicted by theline 1404 which is linear across the input image value range. An inputimage value 1416 is seen to lie between the abscissa value “128” and theabscissa value “255”, and the input image value 1416 will thus, by athree level error diffusion process, map either to a halftone outputvalue of “1”, or to a halftone output value of “2”. For an input imageregion of constant input image value 1416, the proportion of cases inwhich the input image value 1416 maps to “2”, equals the differencebetween the average halftone output of the input image value 1416, andthe halftone output value “1” (ie. 1436). Turning to the lower graph inthe figure, it is seen that the line 1404 has now been tuned, ie.adjusted, to take the form of three linear line segments 1410, 1412 and1414. The input image value 1420 is the same as the input image value1416, however, according to the tuned transfer function, for an inputimage region of constant input image value 1420, the proportion of casesthat the input image value maps to “2”, being the difference between theaverage halftone output of the input image value 1420, and the halftoneoutput value “1”, (ie. 1430), has increased. The effect of tuning thetransfer function, ie. the line 1404, to the set of linear line segments1410, 1412 and 1414, is thus to increase, on average, the utilisation ofsuperintensity output double pixels. The line segment 1414 ishorizontal, clamping all input image values lying between “191”, and“255” on the abscissa 1408, to an output halftoned value of “2”. Thespecific example of tuning shown in FIG. 14 is representative only, andthe feature of tuning a multi-level halftoned output is clearly notlimited thereto.

One method of tuning the halftoning transfer function and adjusting howoften superintensity ink deposition is performed, is provided by simplyremapping image value prior to halftoning, for example, by use of alookup table. An alternative effective method of tuning the halftoningtransfer function, and thereby adjusting how often superintensity inkdeposition is performed, is provided by performing halftoning by errordiffusion with a modified error diffusion table. This method of tuningby use of a modified error diffusion table, is now described.

FIG. 15 shows a process 1900 whereby a tuned error diffusion table isused to print a multi-level halftoned image. In a first step 1902, atuned error diffusion table is prepared, for example using thetechniques described in relation to FIGS. 17 and 18. Thereafter, in astep 1904, an input image signal for halftoning is input into theprocess 1900. In a following step 1906, a corresponding halftone outputfor the input image signal is determined from the table. It is notedthat this will be a “tuned” output, resulting from the aforementionedstep 1902. In a following step 1908, the process 1900 determines whethermore pixels are required to be processed. If there are further pixels inthis category, then the process 1900 is directed in accordance with a“yes” arrow back to the step 1904. If, on the other hand, no more pixelsare required to be processed, then the process 1900 is directed inaccordance with a “no” arrow to a step 1910, at which stage the process1900 is terminated.

FIG. 16 shows, by way of background, example error diffusioncoefficients, where the current pixel is indicated by an asterisk. Errordiffusion halftoning is performed by repeatedly processing a currentpixel, and advancing the current pixel, pixel by pixel, scanline byscanline. Current pixel error is distributed to neighbouring unprocessedpixels shown in the diagram according to the fractions shown. That is,129/256 of a current pixel error is distributed to the next pixel on thesame scanline; and the remainder of the current pixel error isdistributed to 3 pixels on the following scanline. Note that the sum ofthe fractions adds to 1.

For each current pixel, a combined pixel input value is determined asthe sum of the input image value, plus the error distributed to thecurrent pixel. The combined pixel input value is used to index an errordiffusion table 1514 in FIG. 17, or 1602 in FIG. 18, so as to determine(i) halftoned output eg. 1502, 1504 for the current pixel, and (ii)error values eg. 1506, . . . , 1512, to be distributed from the currentpixel to neighbouring unprocessed pixels.

FIGS. 17 and 18 show error diffusion tables 1514, 1602 for 3 levelhalftone output suitable for use in printing super-intensity ink output.In the error diffusion tables 1602, 1514, errors to be distributed areshown as integer values equal to 256 times the fractional error value tobe distributed. The tables have been prepared using the error diffusioncoefficients of FIG. 16 and an algorithm described below. The algorithmcan be used to prepare a table based on a chosen set of image values(eg. “0”, “128”, and “255”, see FIG. 14) corresponding to halftoneoutput values (eg. 0, 1, 2, see FIG. 14), thereby tuning the halftoningtransfer function as previously described in relation to FIG. 14.

The error diffusion table of FIG. 17 has been prepared using imagevalues of 0, 128, 255 corresponding to the 3 halftone output values of0, 1 and 2 (representing, respectively, no dots, 1 dot, and two dots peroutput pixel position). This is equivalent to the arrangement depictedin graph 1424 in FIG. 14. In contrast, the error diffusion table of FIG.18 has been prepared using image values of 0, 128, 191 corresponding tothe 3 halftone output values, this being equivalent to the arrangementdepicted in the graph 1426 in FIG. 14.

Because the image value corresponding to the maximum halftone outputlevel is reduced for the table of FIG. 18 in relation to FIG. 17, use ofthe table of FIG. 18 results in greater use of the super outputintensity ink deposition.

In both tables 1514, 1602, the halftone output 1604 is one of 3 values,each being a pattern of bit values for the 2 output bits: bit o0 (theleast significant bit eg. 1606) and bit o1 (eg. 1608). The halftoneoutput value 2 (bit o0=0; bit o1=1) corresponds to the super intensityink deposition per pixel. Those rows in the tables which are not shown,but are indicated using a row of ellipsis ( . . . ) for example row1610, can be inferred from the preceding row and succeeding row of thetable. Both tables include a column 1612 for “combined pixel input”,which is shown as a sum of (i) an image value, and (ii) an error valuederived from the total error distributed to the current pixel. The tableentries with table index value in the range 320 to 448 (ie. 1516, 1614)in the tables of FIGS. 17 and 18 are not used. These unused tableentries are shown blank in FIGS. 17 and 18 and may be set to zero. Thetable entries e0, e1, e2, e3 (“errors distributed to neighbouringpixels” 1616-1618) with table index value in the range 255 to 319 in thetable of FIG. 18 (ie. 1620) have been determined using clamped pixelerror values. Error clamping is described below, as is an algorithm forpreparation of an error diffusion table based on a chosen set of imagevalues corresponding to the halftoned output levels.

Step 1. For each image value, v, map the image value to the closestimage value corresponding to a halftone output level, out[v]. If thereis a halftone output image value above and below the image value whichare equally close, choose the lower halftone output image value. From vand out[v] determine the error between them: err[v]=v−out[v]. Determinethe minimum and maximum of these error values:err_min=min _(vε0 . . . 255) err[v] anderr_max=max _(vε0 . . . 255) err[v].

Step 2. For each error augmented image value, v_aug, equal to an imagevalue plus an error in the range err_min to err_max, such that the erroraugmented image value is outside the range of image values, map thevalue to the closest image value corresponding to a halftone outputlevel, v_out[v_aug]. Again, if there is a halftone output image valueabove and below the error augmented image value which are equally close,choose the lower halftone output image value. From v_aug and out[v_aug]determine the error between them: err[v_aug]=v_aug−out[v_aug]. Note thatwhen the least image value corresponding to a halftone output level isgreater than 0, or when the greatest image value corresponding to ahalftone output level is less than 255, (as is the case for the table inFIG. 18) err[v_aug] may be outside the range err_min to err_max. Fromerr[v_aug], err_min and err_max, determine a clamped error valuerepresenting the difference between v_aug and out[v_aug] but which is inthe range err_min to err_max.err_clamped[v_aug]=max(err_min, min(err_max, err[v_aug])).

Note that clamping error values ensures that during error diffusionprocessing, the error cannot build up without bound.

Step 3. Fill in the error diffusion table as follows. Firstly zero allentries in the table. For image values, v, the halftone output bitentries are determined as the halftone output level corresponding to theimage value, out[v] and each of the errors distributed to neighbouringpixels is determined by multiplying err[v] by the appropriate errordistribution coefficient, eg. FIG. 16. For error augmented image values,v_aug, outside the range of image values, the halftone output bitentries are determined as the halftone output level corresponding to theerror augmented image value, out[v_aug] and each of the errorsdistributed to neighbouring pixels is determined by multiplyingerr_clamped[v_aug] by the appropriate error distribution coefficient,eg. FIG. 16.

Up to this point, the description has considered the case of a singleblocked nozzle, having fully functional nozzles adjacent thereto.Furthermore, the defective nozzle has thus far been considered to becompletely blocked, and the case of a partially functioning nozzle hasnot been addressed. Where there are two adjacent defective nozzles,compensation by means of either restricted, or full image valueredistribution, as described by the previous C code, can result in theresidual image value of the first defective nozzle being non-zero, evenafter compensation. This is because the image value of the firstdefective nozzle which remains after redistribution to the precedingnozzle cannot be redistributed to the succeeding second defectivenozzle. In order to ensure that the image redistribution method providesa consistent result, the residual image value of a defective nozzle canbe set to zero after defective nozzle compensation has been performed.This is particularly relevant when the defective nozzle is not, in fact,blocked but rather partially functional, producing a random type ofoutput. Thus, for example, such a defective nozzle can sometimes eject arelatively large ink droplet, and at other times can eject a relativelysmall ink droplet, both for the same input firing value. Setting theimage value of such a defective nozzle to zero after image valueredistribution has been performed ensures that any possible residualvalue remaining associated with the defective nozzle does not cause thenozzle to fire.

The description thus far has considered image value redistribution toimmediately neighbouring nozzles, this being the most simple example todescribe. In this type of compensation, surplus ink is deposited by theimmediately neighbouring nozzles in order to compensate for the deficit,or lack of ink deposited by the defective nozzle. A more complexredistribution scheme involving, for example, first and secondneighbouring pixels of a blocked nozzle can also be considered, wherethe expected benefits of such a scheme can justify the increase incomplexity. Alternatively, use of head shading data prepared fromprintout generated by using defective nozzle compensation by image valueredistribution to first neighbour nozzles can provide some compensationfor the surplus local average ink deposition introduced for secondneighbour nozzles.

Defective nozzle compensation by image value redistribution has thus farbeen described on the assumption that defective nozzle data consists ofa 1 bit value for each nozzle, indicating a fully operational state, oralternatively, a fully defective state. Unwanted print artefacts can, insome cases, be further reduced by extending the defective nozzle datadescription to more than a binary description. In such an event, thedegree to which image value is redistributed away from a defectivenozzle can be controlled according to the finer granularity of theprovided defective nozzle data.

The aforementioned description has been directed to defective nozzlecompensation by image value redistribution in respect of fixed printhead systems. Clearly, this can also be applied to shuttlehead printsystems and the like. In respect of shuttlehead print systems, defectivenozzle compensation by image value redistribution is particularlyeffective when the shuttle printer is performing “one pass” printing,since in that case dots of a colour component of a scanline are printedby only a single nozzle.

The method of compensating for a defective printer ink nozzle can bepracticed using a conventional general-purpose computer system 1700,such as that shown in FIG. 19 wherein the processes of, for example,FIGS. 4, 10 and 13 can be implemented as software executing within thecomputer system 1700. In particular, the steps of the method ofcompensating for a defective printer ink nozzle are effected byinstructions in the software that are carried out by the computer. Thesoftware can be divided into two separate parts, one part for carryingout the compensating for a defective printer ink nozzle, and anotherpart to manage the user interface between the latter and the user. Thesoftware can be stored in a computer readable medium, including thestorage devices described below, for example. The software is loadedinto the computer from the computer readable medium, and then executedby the computer. A computer readable medium having such software orcomputer program recorded on it is a computer program product. The useof the computer program product in the computer effects an advantageousapparatus for compensating for defective print nozzles in accordancewith the arrangements of the invention.

The computer system 1700 comprises a computer module 1701, input devicessuch as a keyboard 1702 and mouse 1703, output devices including aprinter 1715 and a display device 1714. A Modulator-Demodulator (Modem)transceiver device 1716 is used by the computer module 1701 forcommunicating to and from a communications network 1720, for exampleconnectable via a telephone line 1721 or other functional medium. Themodem 1716 can be used to obtain access to the Internet, and othernetwork systems, such as a Local Area Network (LAN) or a Wide AreaNetwork (WAN).

The computer module 1701 typically includes at least one processor unit1705, a memory unit 1706, for example formed from semiconductor randomaccess memory (RAM) and read only memory (ROM), input/output (I/O)interfaces including a video interface 1707, and an I/O interface 1713for the keyboard 1702 and mouse 1703 and optionally a joystick (notillustrated), and an interface 1708. for the modem 1716. A storagedevice 1709 is provided and typically includes a hard disk drive 1710and a floppy disk drive 1711. A magnetic tape drive (not illustrated)can also be used. A CD-ROM drive 1712 is typically provided as anon-volatile source of data. The components 1705 to 1713 of the computermodule 1701, typically communicate via an interconnected bus 1704 and ina manner which results in a conventional mode of operation of thecomputer system 1700 known to those in the relevant art. Examples ofcomputers on which the embodiments can be practised include IBM-PC's andcompatibles, Sun Sparcstations or alike computer systems evolvedtherefrom.

Typically, the program of the preferred embodiment is resident on thehard disk drive 1710 and read and controlled in its execution by theprocessor 1705. Intermediate storage of the program and any data fetchedfrom the network 1720 can be accomplished using the semiconductor memory1706, possibly in concert with the hard disk drive 1710. In someinstances, the program can be supplied to the user encoded on a CD-ROMor floppy disk and read via the corresponding drive 1712 or 1711, oralternatively can be read by the user from the network 1720 via themodem device 1716. Still further, the software can also be loaded intothe computer system 1700 from other computer readable medium includingmagnetic tape, a ROM or integrated circuit, a magneto-optical disk, aradio or infra-red transmission channel between the computer module 1701and another device, a computer readable card such as a PCMCIA card, andthe Internet and Intranets including email transmissions and informationrecorded on websites and the like. The foregoing is merely exemplary ofrelevant computer readable mediums. Other computer readable mediums canbe practiced without departing from the scope and spirit of theinvention.

FIG. 20 depicts a block diagram representation 2000 of an apparatus forredistribution of image values from a defective nozzle to neighbouringnon-defective nozzles. The apparatus 2000 has a number of image formingelements 2002, in respect of which, relative desirability data isstored, as depicted by a dashed line 2004, in a memory 2006. Input imagevalues, depicted by an arrow 2012 are input into a processor 2008, aswell as desirability data from the memory 2006. The processor 2008consequently outputs biased image recording signal data, as depicted byan arrow 2010, to the forming elements 2002, which thereby form thedesired image.

The method of compensating for a defective printer ink nozzle canalternatively be implemented in dedicated hardware such as one or moreintegrated circuits performing the functions or sub functions ofcompensating for a defective printer ink nozzle. Such dedicated hardwarecan include graphic processors, digital signal processors, or one ormore microprocessors and associated memories.

INDUSTRIAL APPLICABILITY

It is apparent from the above, that the embodiments of the invention areapplicable to the digital image printing industry. Image valueredistribution provides an effective and computationally simple methodof compensating for defective nozzles. Furthermore, this can be combinedwith existing unevenness correction methods. Restricted image valueredistribution can be combined with cross-colour compensation forimproved defective nozzle compensation. Full image value redistributionand super output intensity printing can be combined with checkerboardquantisation for ease of implementation, and can be used with modifiederror diffusion tables, in order to calibrate how often the super outputintensity ink deposition per pixel is used.

The foregoing describes only some embodiments of the present invention,and modifications and/or changes can be made thereto without departingfrom the scope and spirit of the invention, the embodiments beingillustrative and not restrictive.

1. A method, in printing an image comprising a plurality of imagevalues, of compensating for one or more defective printer nozzles in aplurality of printer nozzles, said method comprising the steps of:biasing, for each first image value associated with a first nozzle, atleast one second image value associated with another nozzle, saidbiasing being dependent upon said first image value and a printingdesirability factor for said first nozzle; halftoning said at least onebiased second image value to form at least one corresponding nozzlefiring value; and printing the image using said at least one nozzlefiring value, thereby reducing print artefacts otherwise caused by theone or more defective nozzles.
 2. The method according to claim 1,wherein the printing desirability factor for said first nozzle providesa measure of one of effectiveness and defectiveness of said firstnozzle.
 3. The method according to claim 1, wherein said biasingcomprises the sub-step of: redistributing one of part of said firstimage value and all of said first image value to said at least onesecond image value associated with the other nozzle being an immediatelyneighbouring nozzle of the same colour.
 4. The method according to claim3, wherein an extent of image value redistribution is dependent upon anallowed operating range of the at least one second image valueassociated with said immediately neighbouring nozzles.
 5. The methodaccording to claim 4, wherein said allowed operating range of said atleast one second image value is between 0% and 100%, wherein 100%represents a maximum intensity for unbiased image values.
 6. The methodaccording to claim 4, wherein said allowed operating range of said atleast one second image value is between 0% and 200%, wherein 100%represents a maximum intensity for unbiased image values, and 200%represents a super-intensity for biased image values.
 7. The methodaccording to claim 1, wherein said biasing further comprises thesub-step of: increasing an image value associated with a correspondingnozzle of another colour.
 8. The method according to claim 6 furthercomprising, prior to printing the image, the sub-step of: mapping thebiased image values from a biased image value range of 0% to 200%, to arange of 0% to 100%.
 9. The method according to claim 8, wherein saidmapping uses checkerboard quantisation, said method comprising the stepsof: dividing said biased image values by 2; and alternately roundingsuccessive divided image values up, and down.
 10. The method accordingto claim 6, wherein, in a multi-level halftoning process, a relationshipbetween an image value and a corresponding average nozzle firing valueis adjusted in order to tune a utilisation of super-intensity printing.11. An image recording apparatus for recording an image comprising aplurality of image values, the apparatus comprising: a plurality offorming elements for forming the image using image recording signals;memory means for storing data for said forming elements indicating therelative desirability of utilising said forming elements for forming theimage; image processing means for computing biased image values usingsaid image values and said data stored in said memory means, wherein useof a particular forming element is biased dependent upon the relativedesirability data of other forming elements; and halftoning means forhalftoning the biased image values to form corresponding said imagerecording signals.
 12. An image recording apparatus for recording animage comprising a plurality of image values, the apparatus comprising:a plurality of forming elements for forming the image using imagerecording signals; memory means for storing data for said formingelements indicating the relative desirability of utilising said formingelements for forming the image; image processing means for computingbiased image values using said image values and said data stored in saidmemory means, wherein use of a particular forming element is biaseddependent upon the relative desirability data of other forming elementsand the image values for the other forming elements; and halftoningmeans for halftoning the biased image values to form corresponding saidimage recording signals.
 13. An image recording apparatus for recordingan image comprising a plurality of image values, the apparatuscomprising: a plurality of forming elements for forming the image usingimage recording signals; memory means for storing data for said formingelements indicating the relative desirability of utilising said formingelements for forming an image; image processing means for redistributingsaid image values based on said data stored in said memory means so asto form redistributed image values the use of which biases use of saidforming elements; and halftoning means for halftoning the redistributedimage values to form corresponding said image recording signals.
 14. Animage recording apparatus for recording an image comprising a pluralityof image values, the apparatus comprising: a plurality of formingelements for forming the image using image recording signals; memorymeans for storing data for said forming elements indicating the relativedesirability of utilising said forming elements for forming the image;image processing means for redistributing said image values based onsaid data stored in said memory means to form redistributed image valuesthe use of which biases the use of said forming elements, wherein theuse of a particular one of said forming elements is thereby biaseddependent upon the relative desirability data of other forming elementsand the image values for the other forming elements; and halftoningmeans for halftoning the redistributed image values to formcorresponding said image recording signals.
 15. The image recordingapparatus according to claim 13 wherein: said image processing means forredistributing said image values does not extend the range of saidredistributed image values.
 16. The image recording apparatus accordingto claim 11 wherein: said apparatus is a colour image recordingapparatus, said plurality of forming elements including plural groups offorming elements respectively corresponding to colour components. 17.The image recording apparatus according to claim 16 wherein: said imageprocessing means includes means for modifying the image values relatingto a colour component based on said image values and based on said dataindicating the relative desirability of utilising said forming elementsrelating to other colour components.
 18. The image recording apparatusaccording to claim 13 wherein: said apparatus is a colour imagerecording apparatus, said plurality of forming elements including pluralgroups of forming elements respectively corresponding to colourcomponents.
 19. The image recording apparatus according to claim 13wherein: said forming elements are capable of recording a super densitybeing greater than any density recorded by said forming elements when noimage values are redistributed by said image processing means; andwherein said image processing means is capable of biasing the use ofsaid forming elements to record said super density.
 20. The imagerecording apparatus according to claim 19 wherein: redistribution ofsaid image values is capable of extending the range of said values. 21.The image recording apparatus according to claim 20 further comprising:image processing means for re-mapping said redistributed image values sothat the range of said values is restored to the range existing prior tosaid redistribution.
 22. The image recording apparatus according toclaim 20 wherein: said image processing means map redistributed imagevalues to the range existing prior to said redistribution by maintainingdistinct local average image values for image regions with differingconstant image values.
 23. The image recording apparatus according toclaim 22 wherein: said image processing means map redistributed imagevalues to the range existing prior to said redistribution bysubstantially dividing redistributed image values by 2 and alternatelyrounding up and rounding down.
 24. The image recording apparatusaccording to any one of claims 20 to 23 wherein the halftoning meansgenerate image recording signals in which the frequency of occurrence ofsuper density recording by forming elements is adjusted according tohalf-toning parameters.
 25. The image recording apparatus according toclaim 24 wherein: said halftoning means generate image recording signalsby error diffusion processing such that the frequency of occurrence ofsuper density recording by forming elements is adjusted according tovalues in an error diffusion table.
 26. The image recording apparatusaccording to claim 13 further comprising: image value forcing meanswhereby image values corresponding to selected forming elements are setto prevent recording; said selected forming elements being determined bysaid data indicating the relative desirability of utilising formingelements.
 27. The image recording apparatus according to claim 13further comprising: (a) memory means storing data for said formingelements based on non-uniformity of the density of a recorded testimage; and (b) correction means for correcting said redistributed imagevalues based on said data stored in said memory means.
 28. The imagerecording apparatus according to claim 13 wherein: each of said formingelements is a forming element for ejecting a liquid drop by film-boilingdue to heat energy.
 29. A method according to claim 1, wherein theprinting step is performed by one of a fixed head print system or ashuttle head print system operating in a single pass mode, wherein aline of dots of a colour component is printed by a single nozzle.